FIGS. 1 and 2 illustrate a prior art downhole tool A which can be suspended from a rig 5 by a wireline 6 and lowered into a well bore 7 for the purpose of evaluating surrounding formations 1. Details relating to apparatus A are described in U.S. Pat. Nos. 4,860,581 and 4,936,139, both assigned to Schlumberger, the entire contents of which are hereby incorporated by reference. The downhole tool A has a hydraulic power module C, a packer module P, and a probe module E. The hydraulic power module C includes pump 16, reservoir 18, and motor 20 to control the operation of the pump 16. Low oil switch 22 also forms part of the control system and is used in regulating the operation of the pump 16.
The hydraulic fluid line 24 is connected to the discharge of the pump 16 and runs through hydraulic power module C and into adjacent modules for use as a hydraulic power source. In the embodiment shown in FIG. 1, the hydraulic fluid line 24 extends through the hydraulic power module C into the probe modules E and/or F depending upon which configuration is used. The hydraulic loop is closed by virtue of the hydraulic fluid return line 26, which in FIG. 1 extends from the probe module E back to the hydraulic power module C where it terminates at the reservoir 18.
The tool A further includes a pump-out module M, seen in FIG. 2, which can be used to dispose of unwanted samples by virtue of pumping fluid through the flow line 54 into the borehole, or may be used to pump fluids from the borehole into the flow line 54 to inflate the straddle packers 28 and 30 (FIG. 1). Furthermore, pump-out module M may be used to draw formation fluid from the borehole via the probe module E or F, and then pump the formation fluid into the sample chamber module S against a buffer fluid therein. In other words, the pump-out module is useful for pumping fluids into, out of, and (axially) through the downhole tool A.
A displacement unit (pump) 92, energized by hydraulic fluid from a hydraulic pump 91, can be configured in various configurations, e.g., to draw from the flow line 54 and dispose of the unwanted sample though flow line 95, or it may be configured to pump fluid from the borehole (via flow line 95) to flow line 54. The pump-out module M can also be configured where flowline 95 connects to the flowline 54 such that fluid may be drawn from the downstream portion of flowline 54 and pumped upstream or vice versa. The pump-out module M has the necessary control devices to regulate the displacement unit 92 and align the fluid line 54 with fluid line 95 to accomplish the pump-out procedure.
With reference now to FIGS. 3A-B and 4A-B, a particular embodiment of the pump-out module M (FIG. 2) using four reversible mud check valves 390 (also referred to as CMV1-CMV4) to direct the flow of the fluid being pumped is depicted. These reversible valves 390 allow the module M to pump either up or down (assuming a vertical borehole section) or in our out (depending on the tool configuration), and utilize a spring-loaded ceramic ball 391 that seals alternately on one of two O-ring seats 393a, 393b to allow fluid flow in only one direction. The O-ring seats are mounted in a sliding piston-cylinder 394, also called a check valve slide or simply a piston slide.
More particularly, FIGS. 3A-B show the respective first and second strokes of the two-stroke operation of the displacement unit 392 with the pump-out module M configured to “pump-in” mode, where fluid is drawn into the module M through a port 346 (e.g., a probe) for communication via a flow line 354. Thus, the solenoid valves S1, S2 are energized in FIGS. 3A-B so as to direct hydraulic fluid pressure to shift piston slides 394 of check valves CMV1 and CMV2 upwardly and shift piston slides 394 of check valves CMV3 and CMV4 downwardly. This results in the upper springs 395a of check valves CMV1 and CMV2 biasing the respective balls 391 against the lower seal seats 393b, and the lower springs 395b of check valves CMV3 and CMV4 biasing the respective balls 391 against the upper seal seats 393a. This allows fluid to flow upwardly through valve CMV2 and downwardly through valve CMV4 (both shown slightly opened) under movement of the displacement unit piston 392p to the left (the first stroke), as indicated by the directional arrows of FIG. 3A. Similarly, this allows fluid to flow upwardly through valve CMV1 and downwardly through valve CMV3 (both shown slightly opened) under movement of the displacement unit piston 392p to the right (the second stroke), as indicated by the directional arrows of FIG. 3B. Sufficient fluid-flowing pressure (e.g., >50 psig) is needed to overcome the respective spring-biasing forces. Solenoid valve S3 is provided to selectively move piston 392p from the position in FIG. 3A to the position in FIG. 3B and back. Solenoid valve S3 is also preferably linked to solenoid valves S1 and S2 to synchronize the timing therebetween.
FIGS. 4A-B, on the other hand, show the respective first and second strokes of the two-stroke operation of the displacement unit 392 with the pump-out module M configured in a “pump-out” mode, where fluid is discharged from the flow line 354 through the port 346 into the borehole. Thus, the solenoid valves S1, S2 have been de-energized in FIGS. 4A-B so as to direct hydraulic pressure to shift piston slides 394 of check valves CMV1 and CMV2 downwardly and shift piston slides 394 of check valves CMV3 and CMV4 upwardly. This results in the lower springs 395b of check valves CMV1 and CMV2 biasing the respective balls 391 against the upper seal seats 393a, and the upper springs 395a of check valves CMV3 and CMV4 biasing the respective balls 391 against the lower seal seats 393b. This allows fluid to flow downwardly through valve CMV1 and upwardly through valve CMV3 (both shown slightly opened) under movement of the displacement unit piston 392p to the left (the first stroke), as indicated by the directional arrows of FIG. 4A. Similarly, this allows fluid to flow downwardly through valve CMV2 and upwardly through valve CMV4 (both shown slightly opened) under movement of the displacement unit piston 392p to the right (the second stroke), as indicated by the directional arrows of FIG. 4B. Again, sufficient fluid-flowing pressure (e.g., >50 psig) is needed to overcome the respective spring-biasing forces.
In each of the FIGS. 3A-B and 4A-B, the check valves having no directional flow arrows are configured such that their respective balls 391 are subjected to fluid pressure assisting the spring-biasing forces, i.e., further compressing each ball against an o-ring seat to maintain a seal. Conversely, when the direction of fluid flow opposes the spring-biasing forces (and overcomes them), a gap is opened between the ball and the seat so as to permit the fluid flow indicated by the directional arrows. The valves open just enough to balance the pressure differential across the opening with the biasing forces provided by the respective springs.
Thus, the fluid being pumped through the tool A flows directly past the O-ring seats 393a,b at various intervals during the two-stoke pumping cycles. Since this fluid (e.g., formation fluid and/or borehole fluid) is often laden with impurities varying from fine mud particles to abrasive debris of various sorts, such flow can and often does produce accelerated wear of the O-ring seats. This wear can shorten the life of the O-rings, and lead to frequent failure of the seals. The following are examples of failures that may occur: 1) the O-ring is gradually worn during the pumping process until it will no longer seal; 2) debris (anything from LCM to heavy oil) gets trapped between the ball and one or both of the O-ring seats; 3) fine particles settle out in the valve cavity, and gradually build up to the point where they will prevent the ball from being able to seal against the seat; and 4) filters that are typically used with such valves are susceptible to plugging. The failure of any one of the four reversible mud check valve seals typically reduces the output of displacement unit 392 down to about half, and the loss of two seals may completely disable the displacement